In a motor vehicle comprising cylinders closed by a cylinder head for the combustion of the fuel, it is known to mount through a bore in the cylinder head a sensor for each cylinder for measuring the pressure of the gases in said cylinder and generating a signal representing the variations of the value of that pressure over time.
This signal is sent to a computer of the vehicle in order to control various parameters. For example, the signal can be used by a computer to decide the optimum time to inject fuel into the cylinder in order to improve the operation of the engine and/or to reduce the emission of pollutants.
As shown in FIG. 1, this pressure measurement sensor 1 comprises a tubular body 2 extending along a longitudinal axis X1 and an element 3 sensitive to the pressure variations mounted in said tubular body 2.
The sensitive element 3 comprises a piezoelectric module 3A for converting a mechanical stress into an electrical voltage, a member 3B for transmitting said mechanical stress to the piezoelectric module 3A and a flange 3C receiving the piezoelectric module 3A and the transmission member 3B.
This flange 3C comprises a tubular portion 3C-1, an elastically deformable membrane 3C-2 and a shoulder 3C-3.
The piezoelectric module 3A and the transmission member 3B are housed in the tubular portion 3C-1 which is itself mounted coaxially in the body 2.
The membrane 3C-2 comprises a circular portion 3C-21 extending transversely in the tubular portion 3C-1 so as to block it and a connecting protuberance 3C-22 extending from the center of said circular portion 3C-21 along the longitudinal axis X1 in order to connect the membrane 3C-2 to the transmission member 3B.
The shoulder 3C-3 extends radially relative to the longitudinal axis X1 from the tubular portion 3C-1 to enable the fixing of the flange 3C to the body 2 by welding at the level of a first weld S1.
The sensor 1 finally comprises a skirt 4 fixed by welding to the shoulder 3C-3 at the level of a second weld S2. This skirt 4 has a conical shape enabling force-fitting of the measurement sensor 1 in a bore in the cylinder head of the vehicle.
When the sensor 1 is operating the membrane 3C-2 is deformed by the effect of the pressure prevailing in the cylinder, thus generating a mechanical stress transmitted by the transmission member 3B to the piezoelectric module 3A that converts it into an electrical voltage representing the pressure prevailing in the cylinder.
A sensor 1 of the above kind has disadvantages, however.
First of all, an assembly of this type necessitates on the one hand laser welding of the shoulder 3C-3 to a body 2 and on the other hand laser welding of the skirt 4 to the shoulder 3C-3, which can prove complex, time-consuming and costly.
Moreover, the cylinder head may be deformed by the effect of the temperature of and stresses in the engine, which can then lead to deformation of the sensor 1 and notably of the membrane 3C-2. Such deformation of the membrane 3C-2 is liable to interfere with the transmission of the mechanical stress between the membrane 3C-2 and the piezoelectric module 3A and therefore with the operation of the sensor 1.
In order partly to alleviate this disadvantage, it is known to mount the membrane 3C-2 at the level of the shoulder 3C-3 in order to stiffen it and thus to limit the impacts on the membrane 3C-2 of deformation of the sensor 1.
However, in this case, when welding the shoulder C3-3 to the body 2 (weld S1) and welding the skirt 4 to the shoulder 3C-3 (weld S2), the heat given off by such welds at the level of the shoulder 3C-3 is transmitted to the membrane 3C-2 and can lead to deformation thereof which again has the aforementioned disadvantages.
Moreover, force-fitting the sensor 1 in the bore in the cylinder head can lead to mechanical deformation of the skirt 4 and the shoulder 3C-3, which are subjected to forces E, such as then to deform the membrane 3C-2, which has the aforementioned disadvantages.
There is therefore a requirement for a pressure sensor that is easy to assemble with no risk of deformation of the membrane.